The Separation of the Two Types of Iodine Molecule and the Photochemical Reaction of Gaseous Iodine with Hexene

Badger, R. M. and Urmston, J. W.
(1930)
The Separation of the Two Types of Iodine Molecule and the Photochemical Reaction of Gaseous Iodine with Hexene.
Proceedings of the National Academy of Sciences of the United States of America, 16
(12).
pp. 808-811.
ISSN 0027-8424.
http://resolver.caltech.edu/CaltechAUTHORS:BADpnas30

Abstract

Soon after Dennison had deduced from the specific-heat curve that ordinary hydrogen gas consists of a mixture of two types of molecule, the so called ortho and para hydrogen, a similar state of affairs in the case of iodine gas was demonstrated by direct experiment by R. W. Wood and F. W. Loomis (1). In brief, these experimenters found that the iodine bands observed in fluorescence stimulated by white light differ from those in the fluorescence excited by the green mercury line λ 5461, which happens to coincide with one of the iodine absorption lines. Half of the lines are missing in the latter case, only those being present which are due to transitions in which the rotational quantum number of the upper state is an even integer. In other words, in the fluorescence spectrum excited by λ 5461 only those lines appear which are due to what we may provisionally call the "ortho" type of iodine molecule.
It is evident than that by irradiating iodine gas with the green mercury line it is possible to selectively activate molecules of the "ortho" type. Furthermore, as shown by these experiments, a molecule of the "ortho" type has an average life time in this form longer than the time it remains in the activated condition before emitting radiation.
It occurred to one of us that these facts might be made use of in effecting a separation of the two molecular types. If some substance is added to the iodine gas with which only the activated molecules will react, one should be able to get rid of them, leaving only the other type of molecule which does not absorb the mercury line.